Sigma delta analog-to-digital converters (ADC's) and digital-to-analog converters (DAC's) have recently come into widespread use with the development of suitable process technology and the increase in digital audio and other applications. Sigma delta converters exhibit excellent linearity and low quantization noise. Because sigma delta ADC's utilize oversampling (sampling at rates in excess of the Nyquist rate), applications of sigma delta ADC's are typically limited to measurement, voice band and audio frequency ranges. Sigma delta ADC's and DAC's are particularly useful in mixed signal integrated circuits in which ADC, digital signal processing and DAC functions are monolithically integrated.
An important component of a sigma delta DAC is an analog output filter which receives an input from a one-bit digital-to-analog converter. The one-bit DAC output is averaged by the output filter to produce an analog output signal. The output filter removes shaped quantization noise in the upper frequency range and rejects images of the Nyquist sampling rate. The output filter typically includes several stages and must meet stringent requirements on amplitude and phase response, high frequency rejection and noise.
Switched capacitor output filters are frequently used for audio and voice band filtering because of their compatibility with integrated circuit processing. Typically, the basic elements of a switched capacitor filter are a capacitor and two switches. A charge is transferred from a voltage source to the capacitor through the first switch on a first clock phase. The first switch is opened, and the charge is transferred from the capacitor to an output through the second switch on a second clock phase. The switches are typically implemented as CMOS transistors. In audio filters, a large value resistor can be replaced with a switched capacitor, resulting in a reduction in area by a factor on the order of 100. An additional advantage of switched capacitor filters which use multiple capacitors is that the filter bandwidth is proportional to capacitance ratios rather than absolute values.
Switched capacitors are frequently used in active filters. In this case, switched capacitor circuits may be used in both the input and feedback paths of an operational amplifier. An exemplary switched capacitor integrator is disclosed by M. Sarhang-Nejad et al in "A True 16-Bit 20 kHz Multibit Sigma Delta ADC With Digital Correction", IEEE 1992 Custom Integrated Circuits Conference Proceedings, pages 16.4.1 to 16.4.4, but the literature contains numerous examples of switched capacitor filters. A switched capacitor filter configuration known as a biquad switched capacitor filter includes two integrator stages connected in series, with feedback from the output of the second stage to the input of the first stage. Biquad switched capacitor filters are disclosed by D. B. Ribner et al in "Biquad Alternatives for High Frequency Switched Capacitor Filters", IEEE Journal of Solid State Circuits, Vol. SC-20, No. 6, December 1985, pages 1085-1095 and by D. Senderowicz et al in "A Family of Differential NMOS Analog Circuits for a PCM Codec Filter Chip", IEEE Journal of Solid State Circuits, Vol. SC-17, No. 6, December 1982, pages 1014-1023.
All of the known prior art biquad switched capacitor filters have had one or more limitations, including relatively large chip area requirements, sensitivity to capacitor mismatches and relatively high noise outputs.